Many polymeric materials are foamed to provide low density articles such as films, cups, food trays, decorative ribbons, and furniture parts. For example, polystyrene beads containing low boiling hydrocarbons such as pentane are formed into lightweight foamed cups for hot drinks such as coffee, tea, hot chocolate, and the like. Polypropylene can be extruded in the presence of blowing agents such as nitrogen or carbon dioxide gas to provide decorative films and ribbons for package wrappings. Also, polypropylene can be injection molded in the presence of blowing agents to form lightweight furniture such as lightweight chairs and table legs.
Polyesters and co-polyesters typically have a much higher density (e.g. about 1.3 g/cc) than other polymers. Therefore, foaming of polyester materials is desirable to decrease weight for their use in making molded parts, films, sheets, food trays, and the like. Such foamed articles also have better insulating properties than non-foamed parts. However, foaming polyesters is difficult. The low melt viscosity and low melt strength of typical poly(ethylene terephthalate) and related co-polyesters create polymer melts which do not adequately retain the bubbles of expanding gases during molding or extrusion operations. Providing polyesters which could be foamed with conventional foaming systems is therefore desirable. Moreover, providing such foamable polyesters that are biodegradable or environmentally non-persistent is even more desirable, especially for one time use items.
Polyesters with acceptable melt strength and melt viscosity for foaming have been prepared by treating preformed linear polyesters with monomeric branching agents such as multi-functional carboxylic acids, anhydrides or polyols to provide branched polyesters. These polyester compositions are disclosed in U.S. Pat. Nos. 3,553,157; 4,132,707; 4,145,466; 4,999,388; 5,000,991; 5,110,844; 5,128,383; 5,134,028; 5,288,764; 5,399,595 and 5,519,066.
Several classes of biodegradable polymers are known in the prior art. For example, cellulose and cellulose derivatives with a low degree of substitution (i.e. less than one) are biodegradable. Polyhydroxyalkanoates (PHA), such as polyhydroxybutyrate (PHB), polycaprolactone (PCL), or copolymers of polyhydroxybutyrate and polyhydroxyvalerate (PHBV), have also been reported to be biodegradable.
Biodegradable polyesters include those prepared from aliphatic diacids or the corresponding carboxylic ester of lower alcohols and diols. The aliphatic polyesters have been used in very few applications mainly because of their low melting points and low glass transition temperatures of generally less than 65.degree. C. and -30.degree. C., respectively. At room temperature, the physical form of many of the aliphatic polyesters is a thick, viscous liquid. Therefore, aliphatic polyesters have not been generally useful. To the contrary aromatic polyesters, such as poly(ethylene terephthalate), poly(cyclohexanedimethanol terephthalate), and poly(ethylene terephthalate-co-isophthalate), are commonly used materials yet are typically very resistant to biodegradation.
U.S. Pat. Nos. 5,292,783 and 5,446,079 disclose block and random linear copolyesters containing both aliphatic and aromatic structures which are biodegradable. However, these polymers are difficult to foam because of their low melt viscosity and melt strength. Heretofore, aliphatic-aromatic copolyesters have not been investigated for their foamability.
Thus, there exists a need in the art for a polyester composition having increased melt strength and viscosity suitable for foaming, yet also being biodegradable so as to be useful in disposable applications. Accordingly, it is to the provision of such an improved foamable polyester that the present invention is primarily directed.